For Short Term Learning Discussions ONLY. This area is for CURSORY questions and connecting with other users ONLY. ALL technical contributions need to be made in the appropriate forums and NOT HERE. All posts are temporary and will be deleted within weeks or months. You should have already search the extensive FAQs in each of the forums before posting here as your question may already be answered.

Note that you need to take into consideration both sides - a high voltage feedthrough is not just an insulated conductor to atmosphere, but to the vacuum side as well. You can "upgrade" the atmospheric side insulation to hold off more, but at some point it will arc on the vacuum side if there is not enough insulation between the conductor and the flange itself. You can certainly improve the voltage hold-off of a 5kv feedthrough to many tens of kV, but I doubt you would be able to effectively run it at those voltages on the vacuum side (depending on your vacuum level, but for the relatively high pressures seen in a fusor it would not hold up long.)

For calculating the atmospheric hold-off voltage, you need to look at the breakdown strength of your oil, and figure out your minimum distances from the HV connection to the chamber. This is a basic estimate, doesn't account for things such as field enhancement effects (if you have any sharp points on the connector.) However, unless you are working with some very, very high voltage systems, much higher than anything you would reasonably run a fusor at, oil immersion of the feedthrough is very unnecessary and over-complicated for this application. Several members have made their own high-voltage feedthroughs very easily, and can be made to hold off whatever voltage you would need. I am looking to build a similar feedthrough that requires no machining, special operations, or oil to make, using standard components, for my 300kV pulsed electron gun build. I would browse the forums more to get an idea of what has been done already.

Generally yes this is correct, but it depends also on how the feedthrough is insulated on the vacuum side, and whether there is plasma or not, in addition to surface defects and contamination. I don't believe Paschen's Law applies the same if the gas is already pre-ionized or as a plasma. If the insulation extends far up along the conductor so there is some distance between the conductor and the adapter face then there will be less issue. But if the insulator stops right near where it breaks through the metal interface then it would have a much higher probability of breaking down. If the surrounding gas is already ionized, then there is more potential for current to flow and conduction to occur through the plasma between the stalk and the feedthrough wall. Surface defects and contamination can also play a significant role in causing breakdown. In a perfect ideal world, there probably wouldn't be too much issue. But it always varies based on the setup. Good rule of thumb is to always try to go with a higher rated feedthrough. I probably would not want to run a 500V feedthrough at 20kV, even if upgraded on the atmosphere side, but a 5-10kV feedthrough may be fine.